This invention relates generally to precipitation and flocculation methods for decontaminating various types of solutions which are contaminated with a variety of contaminants such as heavy metals, radioactive compounds, and organic compounds, using a novel combination of treatment steps. More particularly, this invention relates to methods for decontaminating solutions using aqueous solutions of ferrous sulfate and hydroxides, in combination with flocculants, to precipitate the contaminants and ultimately separate them from solution.
There is increasing concern over the hazards posed by the rising levels of inorganic and organic contaminants within the world's water supplies due to accidental spills, leaks, mining practices and poor disposal practices. Most heavy metal and organic contaminants are toxic to some degree to all life-forms, and can have a deleterious effect on aquatic flora and fauna. In humans, toxic heavy metal poisoning can lead to severe nervous system disorders and can cause death.
In addition, the contamination of drinking water, ground water, soil washing extracting solutions, and leaching solutions presents a further problem in that large volumes of solution typically are affected, making treatment especially problematic. This problem is aggravated in geographical locations where water is in short supply, and the need to recycle is great.
Accordingly, numerous physical, chemical and biological methods for the removal of various contaminants from solutions have been proposed. For example, contaminated water could be treated with aerobic and anaerobic biological purification, electrolysis, membrane filtration, and ion exchange techniques. See "Extractive Methods For Soil Decontamination; A General Survey and Review of Operational Treatment Installations," J. W. Assink, in Contaminated Soil, Edited by J. W. Assink, and W. J. van den Brink, pp. 658, 1986, Martinus Nijhoff Publishers. However, there are problems associated with each of these methods. Contaminated groundwater, soil washing solutions and leaching solutions typically have unique characteristics which adversely effect these available processes. For example, such solutions can contain suspended solids, dissolved humics, or varying compositions, which prevent the successful application of these techniques. In addition, many are not suitable for treatment of large volumes of water due to equipment limitations and cost constraints. For example, ion exchange processes are costly and adversely affected by solids, humics, and even slight changes in solution chemistry. Thus, significant amounts of contamination may remain even after application of the treatment.
Alternatively, it has been suggested that inorganic and organic contaminants in solution can be removed via precipitation techniques, wherein the contaminant is precipitated from solution at a specific pH using carbonates, hydroxides, sulfides, and/or silicates, in conjunction with flocculants or coagulants. Several of these techniques are described in Canter, L. W., and Knox, R. C., Ground Water Pollution Control, Lewis Publishers, Inc., 1985, pp. 110-120; and Willey, B. R., Finding Treatment Options for Inorganics, in WATER/Engineering & Management, Oct. 1987, pp. 28-31. Precipitation methods suffer from another set of drawbacks. For example, sulfide systems are difficult to handle, complex to operate, and often result in high waste volume and harmful residual levels of precipitating agent. Sulfide sludges also are susceptible to oxidation to sulfate when exposed to air, resulting in resolubilization of the metals. Carbonate systems, while relatively easy to operate, are difficult to control and often result in processing problems such as premature plugging of equipment. Hydroxide systems are widely used to remove inorganics because they are the most reliable, and have the added advantages of ease in chemical handling and low volume of sludge. However, the resulting sludge often is gelatinous and difficult to dewater, making treatment, separation, and storage of the contaminated material difficult. Silicate precipitation is not effective on all inorganic contaminants; for example, it does not readily precipitate anionic contaminants or mercury. Therefore, silicate precipitation methods usually are inefficient and ineffective in reducing the level of certain contaminates to environmentally acceptable levels.
What is needed is a simplified, easy-to-operate method of treating large volumes of solutions containing soluble and insoluble heavy metals, radioactive contaminants, and organic contaminants, singly or in combination, which effectively segregates the contaminates from the clean solution and concentrates the contaminated material in a manageable, low volume, concentrated waste stream.
There is a further need for a system that can effectively recover contaminants from extracting solutions used in soil washing processes, which allows recycling of the extracting solution, which requires a minimal amount of equipment and is economical to operate, and which further allows for the processing of recovered contaminants, such as metals, or other salable minerals.
There is also a need for a system which can effectively decontaminate solutions contaminated with both anionic and cationic ions.
There is a further need for a system which can effectively decontaminate solutions containing suspended solids, mobilized soil organics (e.g., humics), and having varying compositional make-ups.